The present invention relates generally to a method of processing semiconductor wafers, and, more particularly, to a method of processing semiconductor wafers in which the wafer is subjected to one or more grinding operations.
Semiconductor wafers are generally prepared from a single crystal ingot, such as a silicon ingot, which is trimmed and ground to have one or more flats for proper orientation of the wafer in subsequent procedures. The ingot is then sliced into individual wafers which are each subjected to a number of processing operations to reduce the thickness of the wafer, remove damage caused by the slicing operation, and to create a highly reflective surface. In conventional wafer shaping processes, the peripheral edge of each wafer is rounded to reduce the risk of wafer damage during further processing. A lapping operation is then performed on the front and back surfaces of the wafer using an abrasive slurry (lapping slurry) and a set of rotating lapping plates. The lapping operation reduces the thickness of the wafer to remove surface damage induced by the slicing operation and to make the opposing side surfaces of each wafer flat and parallel. However, conventional lapping operations are relatively time consuming. For example, a typical lapping operation for wafers having diameters ranging from 200 mm to 300 mm, used to reduce the thickness by about 80 microns, will take approximately 40 minutes to complete.
Upon completion of the lapping operation, the wafers are subjected to a chemical etching operation to further reduce the thickness of the wafer and remove mechanical damage produced by the prior processing operations. One side surface of each wafer (often referred to as the "front" side of the wafer) is then polished using a polishing pad, a colloidal silica slurry (polishing slurry) and a chemical etchant to ensure that the wafer has a highly reflective, damage-free surface. The wafers are typically polished using a two step method of rough polishing for stock removal followed by finish polishing to reduce nonspecularly reflected light (haze). A typical polishing process for wafers having diameters ranging from 200 mm-300 mm reduces the thickness of the wafer by about 10-15 microns and takes approximately 5-15 minutes to complete. A smoother surface may be obtained by including an intermediate polishing operation in which case the entire polishing process takes 30-40 minutes to complete.
The wafers are then cleaned and inspected prior to delivery to the customer for dicing the wafer into semiconductor chips. The wafers must be of sufficient thickness to reduce the risk of surface damage or wafer breakage during packaging and transport. Before cutting the wafer into chips, the wafer is subjected to a conventional backside grinding operation in which the front side of the wafer is covered by a protective covering and laid face down on a table, and the back side of the wafer is ground by a suitable grinding apparatus to substantially reduce the thickness of the wafer.
The conventional method of processing a semiconductor wafer described above, though effective, requires substantial time to complete and also involves significant processing materials cost. The grits used in the lapping and polishing slurries are expensive, and the complexity of the various apparatus used for controlling the slurries and the lapping and polishing pads increases the difficulty in automating the wafer shaping process. In addition, the lapping and polishing operations require considerable time for reducing the thickness of the wafer.
To reduce the processing time, it is known to subject the wafer to a grinding operation in which a rotating grinding wheel having an abrasive surface directly contacts the wafer, without the need for a grit based slurry. The grinding wheel is rotated at high speeds and placed in direct contact with the wafer to grind down the thickness of the wafer. The grinding wheel may be infed into contact with the wafer or the wafer may be infed into contact with the grinding wheel. Water is used during the grinding operation to continually cool the grinding wheel and wafer and remove the silicon swarf.
However, rough grinding operations generally cause deeper crystal lattice damage to the wafer than lapping and polishing operations, resulting in a lesser quality wafer and an increased risk of fracturing of the wafer.